A millimeter wave is an electromagnetic wave (radio wave) with a wavelength between 1 mm and 10 mm. The frequency of a millimeter wave is from 30 GHz to 300 GHz. Sometimes a radio wave with a frequency from 3 GHz and above is alternatively called millimeter wave radio.
Traditionally millimeter waves have been used for the backhaul connection for a mobile network. As the low frequency region becomes crowed with all kinds of radio access services, the millimeter wave becomes increasingly interesting to the research community and the wireless industry for the purpose of establishing the access connection between the access point and the mobile terminal.
There are two special aspects of millimeter wave radio from the perspective of its antenna design. One is that the millimeter wave radio will capture a smaller amount of energy, due to the higher frequency of the waves. Further, millimeter wave radio will have larger noise power, due to the channel bandwidth of millimeter wave radio being normally wider. Millimeter wave radio will suffer from lower signal-to-noise ratio because of the above properties, however, this can be compensated for by higher antenna gains with higher antenna directionality. Because of the smaller wavelength of millimeter waves, the antenna size is smaller for millimeter wave radio. The transmitter and receiver of a millimeter radio can accommodate more antenna elements and narrower beams can hence be produced with a large number of antenna elements resulting in a higher antenna gain. The beam can be formed through a phase control system such that the direction, as well as the beam width can be adjusted.
A narrower beam is beneficial for the purpose of providing higher antenna gains, causing less multi-path fading as well as minimizing cross link interference. To establish and maintain a link between the transmitter and the receiver via narrow beams, it is important to consider the directionality management of the narrow beams for the design, operation and optimization of millimeter wave radio system.
It is known within the art to receive from a user an uplink signal using multiple narrow antenna beams and measuring beam-specific pilot signal power from the uplink signal for all of the beams. The measured pilot signal powers are used to determine which one or ones of the downlink beams is to be used for a downlink signal for the user. This known method suffers from the problem that no solution on how to predicatively compensate for the changes caused by the motion of the mobile terminal is provided.
In millimeter wave based radio systems, the mobile terminal can be moving around with different mobility pattern. There are a number of problems resulting from this movement of the terminal, such as, but not necessarily limited to how the terminal's movement can be tracked and be utilized in the establishment of a high gain millimeter wave link and how the millimeter wave access link can be maintained. Furthermore it is a problem how to decide the next best beam direction from the current best beam direction. Still further, it is a problem how the tracking strategy can be made adaptive when the terminal is in a different environment or with a different movement pattern, such as for example when the terminal is carried by a pedestrian, a high speed vehicle or even if the terminal is a fixed node that is swaying due to a strong wind.